hot water hearter (tank) has drain to outside and is dripping.

My hot water heater is under the house, in a crawl space, and it has a hose connected to it that leads to the outside of the structure. It is a small 1/2 inch hose, or so, and extends about 6" outside of the house. It just sticks outside the stucco and allows water to drip out. I am not sure what it is for, but recently it started dripping. It is releasing water from the hot water heater in a drip. The water is warm. Does anyone know what this means? I put a bucket outside and it filled up a 5 gallon bucket in 24 hours. I am going to have a plumber come, but I am just curious if anyone can help me understand what the issue could be?

Is the hose connected to the drain valve at the bottom of the tank or is it connected to the T/P valve? If it is the drain valve, one of two things are possible. Either the valve somehow has been opened slightly or the valve is faulty. If it is the T/P valve, then there are more possibilities. It could be the T/P is faulty. If you have a closed system, either you do not have a thermal expansion tank or you have one but it has failed. The most likely is the T/P valve.

Should be an easy fix. If the T&P is just bad, it is just a matter of taking off the old one and screwing on the new one. The part nor the labor should be very much. If the T&P is good, but your water pressure is too high, then an expansion tank probably needs to be added. Price for labor varies, but I recently saw someone on here pay about $150 to get one installed.

It must be the T&P.. cuz I have had the house for 7 years with the same water pressure.. and never had any problems.. just this started last week.. Unless somehow I got stronger h20 pressure ..

Do you think turning down the temp on the heater might help. I think I recently turned it up so maybe I caused the problem because now the water is really hot and since then, if I remember correclty, it has been leaking. I put the temp from Med to HOT.

You could replace the T/P valve yourself for about $15, but that may or may not cure the leak. A Thermal expansion tank is needed if you have a closed system. This is created when there is a check valve that prevents excessive pressure from being absorbed by the city main. This check valve can be in a pressure regulator valve which is a bell shaped brass device place in the incoming water supply line. Many cities are now installing water meters that have a check valve to prevent cross contamination of the city water supply. The thermal expansion tank is installed in the incoming water supply line between the PRV and water heater. These cost around $50 to $60. Installation is not difficult if you can sweat copper. All of this said, if you are having a plumber in anyway, you may just want to wait and let him deal with everything. Apparently the water you are losing has a place to go and is not really hurting anything.

The T&P can activate for 2 reasons.
If the water pressure reaches 150-psi
or,
If the temperature reaches 210-degrees F

The other reason being failure of the valve in which case it should be replaced.

Usually on high temperature a large volume of water is released and I do not suspect this to be the cause.
If you have a closed system the thermal expansion may cause a discharge
If a pressure reducing valve has failed it can also cause a discharge

I would check your pressure before doing anything.
If the pressure is high replacing the valve will not do anything.